how do i clean a fish tank?

[zookeeper]

i@ve inherited a protable tv_sized tank with two miserable little goldfish which hasn@t been cleaned out since last july! the water is dark brown and smelly and the gravel at the botttom is thixk with sludge

how do i clean it without killing the poor fish who will surely die of shock?

You need a syphon tube. Suck out water from the bottom of the tank and then top up with a watering can - rainwater is best if you've got a water butt? You can do this in several stages with time in between so they don't get lots of new water all at once. If the tank isn't heated don't add too much cold at once in this weather. You can also clean the glass with a razor blade on a stick thingy if that makes sense. Happy to supply more details if required.
Good luck

thanks leftangle